Electric pump

ABSTRACT

An electric pump includes a stator, a rotor, a shaft, a bearing part, and a pump unit. The stator has a magnetic part, and the stator forms a rotation magnetic field when a coil of the stator is energized. The bearing part has a longitudinal central position that is positioned on an imaginary plane perpendicular to a rotation axis of the rotor. The imaginary plane is positioned within an inner range defined between longitudinal end portions of the magnetic part of the stator.

CROSS REFERENCE TO RELATED APPLICATION

This application is based on and incorporates herein by reference Japanese Patent Application No. 2009-275569 filed on Dec. 3, 2009.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an electric pump.

2. Description of Related Art

A conventional electric motor is known to rotate a shaft, which is rotatably supported by a bearing part, based on a magnetic field generated upon energization to a coil. For example, in an electric oil pump of JP-A-2003-269345, a bearing part of a motor is provided on a housing of the oil pump. Also, in JP-A-2005-160285, bearing parts are provided to both longitudinal ends of a rotor.

The shaft of the electric motor in JP-A-2003-269345 is supported at one position. Also, the bearing part is displaced from the rotor and the stator in the longitudinal direction. As a result, the inclination of the shaft causes unwanted deflection of the rotor in the radial direction, and thereby the performance of the motor becomes unstable. Also, because the inclination of the shaft is relatively large, the clearance between the stator and the rotor needs to be designed substantially large. As a result, the motor output deteriorates disadvantageously. In JP-A-2005-160285, because the shaft is journaled at the longitudinal ends of the rotor, the inclination of the shaft is relatively limited. However, the size of the pump in the longitudinal direction is large disadvantageously.

SUMMARY OF THE INVENTION

The present invention is made in view of the above disadvantages. Thus, it is an objective of the present invention to address at least one of the above disadvantages.

To achieve the objective of the present invention, there is provided an electric pump that includes a stator, a rotor, a shaft, a bearing part, and a pump unit. The stator has a magnetic part, and the stator forms a rotation magnetic field when a coil of the stator is energized. The rotor is rotatably provided at a position radially inward of the stator, and the rotor is rotated about a rotation axis of the rotor by the rotation magnetic field formed by the stator. The shaft is rotatable integrally with the rotor. The bearing part rotatably supports the shaft. The pump unit has a housing provided with an inlet port and a discharge port. The pump unit suctions fluid through the inlet port and discharges fluid through the discharge port based on rotation of the shaft. The bearing part has a longitudinal central position that is positioned on an imaginary plane perpendicular to a rotation axis of the rotor. The imaginary plane is positioned within an inner range defined between longitudinal end portions of the magnetic part of the stator.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention, together with additional objectives, features and advantages thereof, will be best understood from the following description, the appended claims and the accompanying drawings in which:

FIG. 1 is a block diagram illustrating a general configuration of an automatic transmission system that employs an electric pump according to one embodiment of the present invention;

FIG. 2 is an explanatory diagram for explaining an oil pressure circuit of an automatic transmission apparatus that employs the electric pump according to the one embodiment of the present invention;

FIG. 3 is a cross-sectional view illustrating a cross section of the electric pump, which is taken along line of FIG. 4, according to the one embodiment of the present invention;

FIG. 4 is a cross-sectional view illustrating a cross section of the electric pump taken along line IV-IV of FIG. 3;

FIG. 5 is a cross-sectional view illustrating an electric pump of the other embodiment of the present invention; and

FIG. 6 is a cross-sectional view illustrating another electric pump of the other embodiment of the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Electric pumps of embodiments of the present invention will be described with accompanying drawings.

One Embodiment

The electric pump of one embodiment of the present invention is applied to an oil pump that supplies hydraulic oil to an automatic transmission apparatus.

FIG. 1 illustrates a general configuration of a system according to the present embodiment.

An internal combustion engine 80 (hereinafter, referred to as “engine”) serves as a drive force generator for a vehicle, and has a crankshaft (not shown) that is mechanically connected with a drive shaft 82 connected with right and left drive wheels 81. An automatic transmission apparatus 90 is provided to a drive force transmission system that transmits drive force from the crankshaft to the drive wheels 81. The automatic transmission apparatus 90 is provided with an electric pump 10 that is driven based on electric power supplied from a battery 84.

The battery 84 is connected to the electric pump 10, a starter 85, an alternator 86, and an electrical equipment 87. The starter 85 provides initial rotation to the crankshaft of the engine 80. The alternator 86 is mechanically connected with the crankshaft of the engine 80 and converts the fed kinetic energy to electrical energy. Then, the converted electrical energy is charged in the battery 84. The electrical equipment 87 includes an air conditioning apparatus, headlamps, and a fuel injection apparatus. An ECU 89 mainly includes a known microcomputer and executes an idle reduction control, in which the engine 80 is automatically stopped, when the vehicle stops. Also, the ECU 89 executes an automatic starting control for automatically starting the engine 80, which has been stopped under the idle reduction control. Also, the ECU 89 executes an energization control for energizing the electric pump 10. In FIG. 1, lines other than a control line to the electric pump 10 are omitted in order to simplify the drawing.

FIG. 2 illustrates a configuration of an oil pressure circuit of the automatic transmission apparatus 90. The automatic transmission apparatus 90 includes the electric pump 10, a mechanical hydraulic pump 91, a control valve 92, a check valve 94, and multiple friction devices, such as a starting clutch 93.

The mechanical hydraulic pump 91 is driven by the engine 80, and suctions oil stored in an oil pan 98 through a strainer 99. Then, the mechanical hydraulic pump 91 supplies oil to various moving parts (not shown) for lubrication, and also supplies oil pressure to the multiple friction devices through the control valve 92.

The electric pump 10 is provided in parallel to the mechanical hydraulic pump 91. The electric pump 10 is provided to a bypass passage 96 that bypasses the mechanical hydraulic pump 91, and includes a pump unit 12 and a motor unit 13. The pump unit 12 and the motor unit 13 are connected with each other through a shaft 70. The motor unit 13 is connected to a driver 14. The electric pump 10 is driven while the idle reduction control is operated, and supplies oil pressure to the starting clutch 93.

The bypass passage 96 is connected to an oil pressure passage 97 at a connection point that is located downstream of the mechanical hydraulic pump 91 in a flow direction of the pumped fuel. The bypass passage 96 is provided with the check valve 94 at a position between (a) the connection point to the oil pressure passage 97 and (b) the electric pump 10. The check valve 94 opens when oil pressure in the bypass passage 96 exceeds oil pressure in the oil pressure passage 97.

As above, in the present embodiment, when the vehicle stops, the idle reduction control is executed, where the engine 80 is automatically stopped. When the engine 80 is stopped, the mechanical hydraulic pump 91, which is driven by the engine 80, is also stopped. Furthermore, when the mechanical hydraulic pump 91 is stopped, it may be impossible to supply oil to the friction devices, and the oil pressure may reduce if the electric pump 10 is not equipped in an example case. Thus, in the above example case, when the engine 80 is re-started in a state, where oil pressure in the starting clutch 93 is substantially low, the starting clutch may slip and suddenly engage (or lock), resulting in generating shocks during transmission. Thus, in the present embodiment, the electric pump 10 is driven in order to supply oil to the starting clutch 93 through the control valve 92 while the idle reduction control is executed. Thus, oil pressure in the starting clutch 93 is successfully maintained at a sufficient level, and thereby it is possible to reduce the shock generated during the transmission.

The details of the electric pump 10 will be described with reference to FIGS. 3 and 4.

The pump unit 12 of the electric pump 10 is an internal gear pump, and includes a housing 20, an inner rotor 40, and an outer rotor 45.

The housing 20 has a first housing part 21 and a second housing part 31. The first housing part 21 has an inlet port 23 and a discharge port 24. The inlet port 23 is formed at one radial side of the first housing part 21, and the discharge port 24 is formed on the other radial side of the first housing part 21 opposite from the inlet port 23. The first housing part 21 further has a recess 26 formed on a surface of the first housing part 21, which surface contacts the second housing part 31, and the recess 26 is located at a position that corresponds to the position of the shaft 70. The recess 26 receives therein one end portion of the shaft 70. It should be noted that the first housing part 21 does not contact the shaft 70, and thereby the shaft 70 is rotatable without any restriction given by the first housing part 21.

The second housing part 31 has a generally cylindrical shape. A longitudinal end portion of the second housing part 31 adjacent to the motor unit 13 has a tubular portion 32 having a hollow cylindrical shape. The other longitudinal end portion of the second housing part 31 opposite from the tubular portion 32 has a receiving chamber 34 that receives therein the inner rotor 40 and the outer rotor 45. Also, the second housing part 31 has a bearing hole 35 that rotatably supports the shaft 70. The bearing hole 35 serves as a second bearing part. There is a clearance formed between the bearing hole 35 and the shaft 70, and the clearance is supplied with oil leaked from an oil chamber 49 (described later) in order to reduce the sliding resistance caused by rotation of the shaft 70. Also, there is an oil seal 33 provided in the longitudinal direction of the second housing part 31 between a bottom portion of the tubular portion 32 and a first bearing part 75 (described later). Thus, it is possible to prevent the flow of oil into the motor unit 13.

The first housing part 21 is fixed to the second housing part 31 through bolts 30. The second housing part 31 has an O-ring groove 37 formed on a surface of the second housing part 31 that contacts the first housing part 21. The O-ring groove 37 receives therein an O-ring 371 such that a clearance between the first housing part 21 and the second housing part 31 is seal by the O-ring 371. Also, one side of the second housing part 31 that is remote from the first housing part 21 is covered by a cover 39 that receives therein the motor unit 13. The second housing part 31 further has another O-ring groove 38 formed on another surface of the second housing part 31 that contacts the cover 39. The O-ring groove 38 receives therein an O-ring 381 such that air-tightness between the second housing part 31 and the cover 39 is effectively achieved. It should be noted that the second housing part 31 and the cover 39 constitutes a housing of the pump unit 12 and also constitutes a housing of the motor unit 13.

The inner rotor 40 and the outer rotor 45 are both made of a material, such as a sintered metal, which material has a substantial abrasion resistance. The inner rotor 40 and the outer rotor 45 are rotatably received in a space defined by the first housing part 21 and the receiving chamber 34 of the second housing part 31.

The inner rotor 40 (interior gear) has a shaft hole 41 formed at a radial center of the inner rotor 40. The shaft hole 41 has two flat surfaces 42 that extends in a direction generally parallel to the longitudinal direction of the shaft hole 41. The two flat surfaces 42 are generally parallel to each other. The two flat surfaces 42 are connected by arc surfaces as shown in FIG. 4. The shaft hole 41 is fitted with a fitting part 71 of the shaft 70 such that the inner rotor 40 is rotatable integrally with the shaft 70. Also, the inner rotor 40 has seven external teeth 44 formed at an outer periphery of the inner rotor 40.

The outer rotor 45 (exterior gear) is provided at a position radially outward of the inner rotor 40 and has a generally hollow cylindrical shape. The outer rotor 45 has eight internal teeth 46 formed at an inner periphery of the outer rotor 45, and the internal teeth 46 are meshed with or engaged with the external teeth 44 of the inner rotor 40. The inner rotor 40 has a rotation center that is eccentric to a rotation center of the outer rotor 45, and there is a space 48 formed between the inner rotor 40 and the outer rotor 45.

The space 48 is communicated with the oil chamber 49 that is formed to extend between the first housing part 21 and the second housing part 31. The oil chamber 49 is communicated with the inlet port 23 and the discharge port 24. Thus, the inlet port 23 and the discharge port 24 are communicated with each other through the oil chamber 49 and the space 48.

The motor unit 13 includes a stator 50 and a rotor 60.

The stator 50 has a magnetic part 51 and an insulator 53. The magnetic part 51 is formed by laminating multiple magnetic plates onto one another. The insulator 53 is made of a non-magnetic material, and is located at the longitudinal ends of the magnetic part 51. The insulator 53 is wound with a coil. When the coil is energized, the magnetic part 51 of the stator 50 is formed with a rotation magnetic field.

The rotor 60 has a cylindrical cup shape that opens to the pump unit 12, and is rotatably provided at a position radially inward of the stator 50. The rotor 60 has a bottom portion 61 and a peripheral wall 64 that is provided at an outer periphery of the bottom portion 61. The bottom portion 61 has a hole 62 at a radial center thereof. The hole 62 receives therein an end portion of the shaft 70, which end portion is remote from the fitting part 71 of the shaft 70, and the end portion of the shaft 70 is press-fitted into the hole 62 in a fixed manner. As a result, the rotor 60 is rotatable about a rotation axis of the rotor 60 integrally with the shaft 70. The peripheral wall 64 has a radially outer surface that is attached with a magnet 65. In the present embodiment, the magnet 65 of the rotor 60 has a length measured in the longitudinal direction of the rotor 60, and the length of the magnet 65 generally coincides with a length of the magnetic part 51 of the stator 50 measured in the longitudinal direction of the stator 50.

Also, a receiving space 68 is formed by a radially inner surface 67 of the peripheral wall 64 of the rotor 60, and the receiving space 68 receives therein an end of the tubular portion 32 of the second housing part 31. It should be noted that there is a clearance formed between the radially inner surface 67 of the rotor 60 and the tubular portion 32 of the second housing part 31. Thereby, the radially inner surface 67 is spaced away from the tubular portion 32. In other words, the radially inner surface 67 does not contact the tubular portion 32.

The shaft 70 has a generally cylindrical shape. The shaft 70 has one longitudinal end portion that is received by the shaft hole 41 of the inner rotor 40 and the other longitudinal end portion that is press-fitted into the rotor 60. The end portion of the shaft 70 adjacent to the inner rotor 40 is cut such that the fitting part 71 is formed to have two cut surfaces 72 that extend in the longitudinal direction of the shaft 70. The two cut surfaces 72 are formed by machining to extend in parallel to each other. A distance between the two cut surfaces 72 generally coincides with a distance between the two flat surfaces 42 formed at the inner rotor 40. In other words, a dimension of the fitting part 71 measured in the radial direction between the two cut surfaces 72 generally coincides with a dimension of the shaft hole 41 of the inner rotor 40 measured in the radial direction between the two flat surfaces 42. Each cut surface 72 is fitted with the respective flat surface 42 such that the fitting part 71 is fitted into the shaft hole 41. As a result, it is possible to limit the rotation of the shaft 70 relative to the inner rotor 40. Thereby, the shaft 70 rotates integrally with the inner rotor 40.

The shaft 70 is journaled at two positions by (a) the first bearing part 75 and (b) the bearing hole 35 of the second housing part 31.

The receiving space 68 is formed at a position radially inward of the rotor 60, and the receiving space 68 receives therein the first bearing part 75. The first bearing part 75 rotatably supports the shaft 70. The first bearing part 75 is a ball bearing having an inner race 76, an outer race 77, and balls 78. The shaft 70 is press-fitted into the inner race 76. Also, the outer race 77 forms an outer casing of the first bearing part 75 and is press-fitted into the tubular portion 32 of the second housing part 31 that is received within the receiving space 68. In other words, the first bearing part 75 is provided within a space formed at a position radially inward of the rotor 60. The balls 78 are held between the inner race 76 and the outer race 77 by a holder. Central positions O of the balls 78 coincide with a longitudinal central position of the first bearing part 75 in the longitudinal direction of the first bearing part 75. For example, the central positions O of the balls 78 and the longitudinal central position of the first bearing part 75 are located on an imaginary plane that is perpendicular to the rotation axis of the rotor 60. In other words, an imaginary line L3 (shown by a dashed and double-dotted line in FIG. 3), which indicates the longitudinal central position of the first bearing part 75 in the longitudinal direction of the first bearing part 75, includes the central positions O of the balls 78.

In FIG. 3, an imaginary line L1 indicates a position of a longitudinal end portion 55 of the magnetic part 51 of the stator 50, which end portion 55 is located adjacent to the first housing part 21. In other words, the imaginary line L1 is defined to extend along a plane of the longitudinal end portion 55 of the magnetic part 51, which is adjacent to the first housing part 21. Also, an imaginary line L2 indicates a position of the other longitudinal end portion 56 of the magnetic part 51, which end portion 56 is located remote from the first housing part 21. In other words, the imaginary line L2 is defined to extend along a plane of the other longitudinal end portion 56 of the magnetic part 51, which is remote from the first housing part 21. Both of the imaginary line L1 and the imaginary line L2 are shown by dashed and double-dotted lines. Also, the imaginary line L3 shows the longitudinal central position of the first bearing part 75 in the longitudinal direction. As shown in FIG. 3, the imaginary line L3 is positioned within a range in the longitudinal direction of the first bearing part 75 (or in the longitudinal direction of the rotor 50), which range is defined between the imaginary line L1 and the imaginary line L2. More specifically, in the present embodiment, the imaginary line L3 is positioned at a middle point of the imaginary line L1 and the imaginary line L2 in the longitudinal direction of the first bearing part 75. Also, the bearing hole 35 is positioned out of the range in the longitudinal direction of the first bearing part 75.

It should be noted that in the present embodiment, the range defined between the imaginary line L1 and the imaginary line L2 corresponds to an “inner range”. Thus, the inner range is defined between the longitudinal end portions 55, 56 of the magnetic part 51 of the stator 50. Also, for example, the first bearing part 75 has the longitudinal central position that is positioned on an imaginary plane perpendicular to the rotation axis of the rotor 60, and the imaginary plane is positioned within the inner range along the rotation axis of the rotor 60. In the above condition, the imaginary line L3 in FIG. 3 corresponds to the imaginary plane, for example.

Also, a clearance defined in a radial direction between the shaft 70 and the bearing hole 35 of the second housing part 31 is designed to be smaller than a total of (a) a clearance defined in the radial direction between the inner race 76 and the balls 78 and (b) another clearance defined in the radial direction between the outer race 77 and the balls 78. Thereby, an inclination amount, by which the shaft 70 may be angled relative to the bearing hole 35, is smaller than an inclination amount, by which the shaft 70 may be angled relative to the first bearing part 75, for example.

It should be noted that in the present embodiment, an internal space of the first bearing part 75 corresponds to the total of (a) a clearance defined in the radial direction between the inner race 76 and the balls 78 and (b) another clearance defined in the radial direction between the outer race 77 and the balls 78. Also, the internal space of the first bearing part 75 corresponds to a “first displaceable amount, by which the shaft 70 is displaceable in the radial direction of the rotor 60 at the first bearing part 75”. Also, the clearance defined in the radial direction between the bearing hole 35 and the shaft 70 corresponds to a “second displaceable amount, by which the shaft 70 is displaceable in the radial direction at the second bearing part 35”. Thereby, the second displaceable amount is smaller than the first displaceable amount.

Operation of the electric pump 10 will be described.

When the coil wound on the insulator 53 of the stator 50 is energized, the rotation magnetic field is formed on the magnetic part 51 of the stator 50. Due to the formed rotation magnetic field, the rotor 60, the shaft 70, and the inner rotor 40 rotate integrally with each other. Also, the outer rotor 45 rotates upon the rotation of the inner rotor 40. When the inner rotor 40 and the outer rotor 45 rotate, the meshing (or the engagement) of the external teeth 44 with the internal teeth 46 changes continuously, and thereby the volume of the space 48 formed between the inner rotor 40 and the outer rotor 45 changes continuously. Then, a void is created in one segment of the space 48 separated by the rotors 40, 45 when the volume of the one segment of the space 48 increases, and thereby oil is suctioned into the one segment through the inlet port 23. At the same time, the volume of the other segment of the space 48 decreases, and oil in the other segment of the space 48 is discharged through the discharge port 24.

In the present embodiment, the shaft 70 is journaled at the two positions by the first bearing part 75 and the bearing hole 35. Also, the longitudinal central position of the first bearing part 75 generally coincides with the longitudinal central position of the magnetic part 51 of the stator 50 in the longitudinal direction of the stator 50. For example, the longitudinal central position of the first bearing part 75 and the longitudinal central position of the magnetic part 51 are positioned on the same imaginary plane that is perpendicular to the rotation axis of the rotor 60. In other words, the longitudinal central position of the first bearing part 75 and the longitudinal central position of the magnetic part 51 of the stator 50 are located on the same imaginary plane perpendicular to the rotation axis of the rotor 60. As a result, because the longitudinal central position of the first bearing part 75 is located at a position that generally coincides, in the longitudinal direction of the stator 50, with the central position of the rotation magnetic field formed by the stator 50, the inclination of the shaft 70 when rotated is effectively limited.

Also, the bearing hole 35 is positioned longitudinally out of the inner range defined between the longitudinal ends of the magnetic part 51 of the stator 50. Thus, the shaft 70 is journaled by the first bearing part 75 at a position within the rotation magnetic field, and is simultaneously journaled by the bearing hole 35 at a position out of the rotation magnetic field. As a result, it is possible to further effectively limit the inclination of the shaft 70 when the shaft 70 rotates. Also, the clearance in the radial direction between the bearing hole 35 of the second housing part 31 and the shaft 70 is designed to be smaller than the total of (a) the clearance in the radial direction between the inner race 76 and the balls 78 and (b) the other clearance in the radial direction between the outer race 77 and the balls 78. As a result, it is possible to effectively limit the inclination of the shaft 70, and it is also possible to improve the flexibility in designing of the internal space formed within the first bearing part 75.

As above, the longitudinal central position of the first bearing part 75 is positioned within the inner range defined between the longitudinal ends 55, 56 of the magnetic part 51 of the stator 50. In other words, the longitudinal central position of the first bearing part 75 is positioned between the imaginary line L1 and the imaginary line L2 in the longitudinal direction of the first bearing part 75. As a result, the shaft 70 is journaled at the position that is the center of the rotation magnetic field formed by the stator 50 when the coil of the stator 50 is energized, and thereby it is possible to effectively limit the inclination of the shaft 70 while the shaft 70 rotates. Because the inclination of the shaft 70 is limited, the unwanted deflection of the rotor 60 in the radial direction is reduced, and thereby the motor performance becomes more reliable. Also, because the unwanted deflection of the rotor 60 in the radial direction is reduced, it is possible to effectively reduce the clearance between the stator 50 and the rotor 60. Thus, it is possible to relatively improve the output of the motor. Also, it is possible to reduce the size of the electric pump in the longitudinal direction to be smaller than the size of the electric pump in the longitudinal direction of a comparison case, where the rotor 60 is supported at two positions at the longitudinal ends thereof. Specifically, in the present embodiment, because the first bearing part 75 is provided within the receiving space 68 that is formed at the position radially inward of the rotor 60, it is possible to effectively utilize the space. As a result, it is possible to effectively reduce the size of the electric pump.

The longitudinal central position of the first bearing part 75 generally corresponds to the central position of the magnetic part 51 of the stator 50 in the longitudinal direction. Due to the above, the shaft 70 is journaled by the first bearing part 75 at the generally central position of the magnetic field formed by the stator 50. As a result, it is possible to effectively limit the inclination of the shaft 70.

In the present embodiment, the shaft 70 is journaled at two positions by the first bearing part 75 and the bearing hole 35 of the second housing part 31. More specifically, the bearing hole 35 is positioned out of the inner range in the longitudinal direction of the bearing hole 35, which range is defined between the longitudinal ends 55, 56 of the magnetic part 51 of the stator 50. As a result, the shaft 70 is journaled at two positions inside and outside of the rotation magnetic field formed by the stator 50, and thereby it is possible to further effectively limit the inclination of the shaft 70.

Also, the clearance in the radial direction defined between the bearing hole 35 and the shaft 70 is designed to be smaller than the clearance in the radial direction defined between (a) the inner race 76 and the outer race 77 and (b) the balls 78. In the above, the clearance in the radial direction defined between (a) the inner race 76 and the outer race 77 and (b) the balls 78 corresponds to the gap inside the first bearing part 75. As a result, the inclination of the shaft 70 while the shaft 70 rotates is determined by the clearance between the shaft 70 and the bearing hole 35 that is positioned out of the inner range. Thereby, it is possible to further effectively limit the inclination of the shaft 70. Also, it is possible to improve the flexibility in design of the clearance in the radial direction at the first bearing part 75.

Other Embodiment

In the above embodiment, the longitudinal central position of the first bearing part 75 generally coincides with the longitudinal central position of the magnetic part 51 of the stator 50. In the other embodiment of the present embodiment, the longitudinal central position of the first bearing part 75 may be located at any position within the inner range defined between the longitudinal end portions 55, 56 of the magnetic part 51 of the stator 50 as shown in FIGS. 5 and 6. It should be noted that similar components of the pump of the present embodiment, which are similar to the components of the pump of the above embodiment, will be designated by the same numerals, and the explanation thereof will be omitted. In the example illustrated in FIG. 5, an imaginary line L4 shows the longitudinal central position of the first bearing part 75. The imaginary line L4 (corresponding to the imaginary plane) is positioned in the longitudinal direction of the first bearing part 75 between the imaginary line L1 and the imaginary line L2. Also, in an example of FIG. 6, an imaginary line L5 (corresponding to the imaginary plane) is defined to indicate a longitudinal central position of the first bearing part 75. The imaginary line L5 is positioned between the imaginary line L1 (corresponding to the longitudinal end portion 55 of the stator 50) and the imaginary line L2 (corresponding to the other longitudinal end portion 56 of the stator 50), and generally coincides with the imaginary line L1. Even in the above configuration, because the shaft 70 is journaled by the first bearing part 75 within the rotation magnetic field formed by the stator 50, it is possible to effectively limit the inclination of the shaft 70.

In the above embodiment, the second bearing part 35 is provided to the second housing part 31. More specifically, the second bearing part 35 is provided to the second housing part 31 at a position adjacent to the pump unit 12. In the other embodiment, the second bearing part 35 may be provided to the other side of the second housing part 31 remote from the pump unit 12. It should be noted that if a distance between the second bearing part 35 and the first bearing part 75 is greater, it is possible to more effectively limit the inclination of the shaft 70. Although there are the first bearing part 75 and the second bearing part 35 in the above embodiment, there may be three or more bearing parts alternatively. When multiple bearing parts are provided, the longitudinal central position of at least one of the multiple bearing parts should be positioned within the inner range in the longitudinal direction of the shaft 70, which range is defined between the longitudinal end portions 55, 56 of the magnetic part 51 of the stator 50, and the other bearing part may be located at any position that is within or out of the inner range in the longitudinal direction of the shaft 70.

In the above embodiment, the pump is the internal gear pump, and the number of teeth of the inner rotor 40 of the internal gear pump is 7, and the number of teeth of the outer rotor 45 of the internal gear pump is 8. However, the number of teeth of the inner rotor 40 and the number of teeth of the outer rotor 45 may be changed accordingly to the discharge amount as required. In the above alternative case, the number of the internal teeth 46 of the outer rotor 45 should be set greater by one than the number of the external teeth 44 of the inner rotor 40. Also, the pump is not limited to the above internal gear pump. However, the pump may be any rotation pump provided that the rotation pump pumps fluid based on the rotation of the shaft. Also, although the electric pump is an oil pump that pumps oil in the above embodiment, pumped fluid is not limited to oil, and thereby the electric pump may alternatively be a water pump.

The motor unit of the above embodiment is a surface permanent magnet (SPM) motor that has the magnet attached to the outer periphery of the rotor. However, the motor unit is not limited to the SPM motor. The motor unit may be other motor different from the SPM motor, such as an interior permanent magnet (IPM) motor. Also, the number of magnetic poles of the magnet of the rotor may be any number.

Also, in the above embodiment, the dimension of the stator 50 in the longitudinal direction coincides with the dimension of the rotor 60 in the longitudinal direction. However, the dimension of the stator 50 in the longitudinal direction may be different from the dimension of the rotor 60 in the longitudinal direction.

Also, in the above embodiment, the electric pump is applied to the automatic transmission apparatus of the vehicle. However, in the other embodiment, the electric pump may be applied to anything in any other field provided that the electric pump pumps fluid.

The present invention is not limited to the above embodiments. The present invention may be modified in various forms provided that the modification does not deviate from gist of the invention.

Additional advantages and modifications will readily occur to those skilled in the art. The invention in its broader terms is therefore not limited to the specific details, representative apparatus, and illustrative examples shown and described. 

1. An electric pump comprising: a stator that has a magnetic part, the stator forming a rotation magnetic field when a coil of the stator is energized; a rotor that is rotatably provided at a position radially inward of the stator, the rotor being rotated about a rotation axis of the rotor by the rotation magnetic field formed by the stator; a shaft that is rotatable integrally with the rotor; a bearing part that rotatably supports the shaft; and a pump unit that has a housing provided with an inlet port and a discharge port, the pump unit suctioning fluid through the inlet port and discharging fluid through the discharge port based on rotation of the shaft, wherein: the bearing part has a longitudinal central position that is positioned on an imaginary plane perpendicular to the rotation axis of the rotor; and the imaginary plane is positioned within an inner range defined between longitudinal end portions of the magnetic part of the stator.
 2. The electric pump according to claim 1, wherein: the stator has a longitudinal central position that is positioned on the imaginary plane, on which the longitudinal central position of the bearing part is positioned.
 3. The electric pump according to claim 1, wherein: the bearing part is a first bearing part, the electric pump further comprising a second bearing part that rotatably supports the shaft, wherein: the second bearing part has a longitudinal central position that is positioned on the other imaginary plane perpendicular to the rotation axis of the rotor, the other imaginary plane being positioned out of the inner range along the rotation axis of the rotor.
 4. The electric pump according to claim 3, wherein: the magnetic part of the stator has a longitudinal central position that is positioned on the imaginary plane, on which the longitudinal central position of the first bearing part is positioned.
 5. The electric pump according to claim 3, wherein: the shaft is displaceable in a radial direction of the rotor at the first bearing part by a first displaceable amount; and the shaft is displaceable in the radial direction at the second bearing part by a second displaceable amount that is smaller than the first displaceable amount.
 6. The electric pump according to claim 1, wherein: the pump unit includes: an inner rotor that has external teeth formed at an outer periphery thereof, the inner rotor being rotatable integrally with the shaft; and an outer rotor that has internal teeth formed at an inner periphery thereof, the internal teeth being meshed with the external teeth; and the housing rotatably receives therein the inner rotor and the outer rotor. 